In the normal operation of a diesel engine, atmospheric air is first compressed in the combustion chamber of the engine to a pressure of about 500 PSI. Compression of the air raises its temperature to about 1,000.degree. F. Diesel fuel is then injected to the compressed hot air through a fuel injection nozzle. The fuel is atomized in the combustion chamber where it rises to its auto ignition temperature, resulting in the spontaneous ignition, burning, and expansion of the gases in the chamber. The expansion of the combustion products drives the cylinder downwardly thereby providing the power stroke of the engine.
In order for a diesel engine to operate efficiently, i.e., with minimum fuel consumption at maximum power, it is typically operated under air to fuel ratios which produce exhaust gases that contain large amounts of oxygen and usually only minimal amounts of unburned hydrocarbons. Unfortunately, operating a diesel engine for maximum power and efficiency also results in conditions that raise the peak operating temperatures and therefore results in the formation of compounds of nitrogen and oxygen known as NO.sub.x compounds.
One method for lowering the concentration of NO.sub.x compounds in exhaust gas emissions is, of course, to bring the exhaust gas into contact with a catalyst capable of reducing the amount of NO.sub.x species in the gas stream. However, for catalysts known to be effective in a diesel exhaust environment, catalytic deNO.sub.x is usually more effective when reducing species are present in the exhaust gas. In order to generate these species, the engine is operated at conditions of low peak temperature which are conditions that are directly opposed to what is desired from the stand point of overall efficient engine operation.
One way that the concentration of oxygenates, unsaturates, unsaturated oxygenates and mixtures thereof can be increased in the exhaust is by direct injection of a hydrocarbon into the diesel engine's cylinder during the engine's expansion (power) stroke. This is referred to as secondary injection. Many deNO.sub.x catalysts capable of using such hydrocarbon to reduce NO.sub.x species in the exhaust stream are more effective when the exhaust stream's NO.sub.2 :NO ratio can be controlled to provide an optimum ratio, depending on the particular catalyst employed.
Consequently, there is a need for controlling the NO.sub.2 :NO ratio among the NO.sub.x exhaust species in a diesel engine that employs secondary injection.